A motor-driven vehicle such as an electric vehicle (EV), a plug-in hybrid vehicle (PHEV) and the like is propelled by a motor utilizing power stored in a secondary battery as a source of motive power for driving the wheels. In this motor-driven vehicle, braking force is generated by executing regenerative braking which is attained by operating a motor as a generator at the time of speed reduction with recovery of kinetic energy in the form of electrical energy. As seen, in the electric vehicle, an electric braking system is used for providing part of the braking force.
Among conventional motor-driven vehicles, there is a vehicle of the type having an engine for travel and a motor and a secondary battery, wherein a canister for absorbing fuel evaporated from the engine is equipped with a heater and regenerative power is fed to the heater to purge the evaporated fuel for supply to the intake system of the engine when regenerative power cannot be used for charging the secondary battery because the secondary battery is in a fully charged condition. This is the situation exemplified in Japanese published patent application 1999-343890.
Japanese published patent application 2008-195315 discloses a motor-driven vehicle of the type having a secondary power source which is charged using at least either power from the engine or from an external power source, wherein the battery is kept charged using the power from the engine when the temperature of the charged battery is equal to or less than a predetermined value after charging of the battery by the external power source is completed.
Now, when a secondary battery is being charged, it would appear that it is sufficient to periodically measure the state of charge of the battery during charging, and then finish charging when the battery is incapable of receiving any further charge. However, this is not sufficient to meet the requirements of regenerative braking which require that the battery be in a suitable state of charge to allow it to receive a further charge, although small, during regenerative braking. Since braking capability using an electric regenerative braking system depends on the state of the secondary battery (state of charge and temperature), a problem arises making it necessary to employ an expensive and complicated system of the cooperative regenerative braking system type. Especially, when the voltage of the secondary battery has been increased to near the level corresponding to a chargeable upper limit, charging capability of the secondary battery using regenerative power becomes diminished.
For this reason, another problem arises in that, depending on the state of charge of the secondary battery, braking force using regenerative braking which is responsible for a part of the braking of the electric vehicle cannot always be deployed to its best advantage.